1. Field of the Invention
The present invention relates to fluid handling using concentric tubes. More specifically, the invention relates to using heated water to insulate and heat a liquid source gas line to prevent the gas vapor from condensating in the gas line downstream of the source.
2. Description of the Related Art
In the fabrication of LSI and VLSI semiconductor devices, many of the etch steps required have to be done with a dry etch process. Most metal etch and many poly etch processes require the use of a chlorine based chemistry. To optimize the process, there are several gasses that are used which are called liquid source gases. This means that at atmospheric pressure, the gas is in a liquid state. When the liquid is put in a container, and a vacuum source is applied, the vacuum will pull the vapor or gas off the top of the liquid. When a sufficient reduction in pressure is obtained, there will be enough gas in the line to supply the etch reactor.
The problem with this system is that as the gas vapor gets further away from the liquid source, it becomes more vulnerable to a temperature condensation effect. In other words, the gas turns back into a liquid in the gas line. This in turn blocks the gas line and restricts any gas from arriving at the etch reactor.
The key in preventing this from happening is to keep the bottle or tank which contains the liquid gas source cooler than any other point in the gas distribution system. The conventional way to do this involves two different methods. The first method involves placing a coolant jacket around the liquid source container. The temperature is controlled by a chiller. This would result in being able to keep the liquid cooler than any other point in the gas distribution system.
The other method of controlling the temperature of the gas delivery system involves keeping the gas lines from the outlet of the liquid source container to the rest of the gas distribution system warmer than the liquid source container. This is conventionally done by either insulating the gas lines to keep cool air from blowing on them or by wrapping the lines with an electrical heater line. The temperature of this line is controlled by either a transformer, or just plugged into a wall socket. Both the heater line and the insulator can be used together to ensure that the gas lines stay warmer than the liquid source.
There are several problems associated with using a heater line. First, the length of heater line needed can be excessive since the best way to insure that the gas line is heated properly is to wrap the heater line around the gas line evenly. Not only is the line lengthy, it is also rather unsightly. The second problem with using a heater line is that the line becomes brittle after a period of time. This could result in an open electrical line. Also, it is sometimes hard to keep the temperature from getting too high. If insulation is used around the heater line, sometimes the high temperature will melt the insulation.
New federal regulations require that double containment gas lines be used for all corrosive gasses. This applies only to new fab construction and remodeling. These gas lines have an inner and outer shell. The inner shell is where the gas flows. The outer shell is just a safety backup in case the inner shell develops a leak.
Babcock & Wilcox' U.S. Pat. No. 3,098,023, to Schluderberg, describes a nuclear reactor primary coolant containment system in which pipe sections have annular shells. The shells are used to pressure test the system. They also provide a fluid containment feature so that a loss of coolant accident is unlikely to occur unless someone shuts off the wrong valve. The concentric annular pipe is used for redundancy and for a pressure test function.
The Boise Warm Springs Water District municipal hot springs system uses concentric pipes for all new underground supply connections. The inner pipe is iron, which is passivated from corrosion by the chemical balance of the water. Previous single walled pipes were iron, and these did not corrode from the inside. The outside of the single walled pipes were subject to corrosion, which resulted from electrolytic action in the soil, accelerated by the heat from the hot springs water. Coated pipe was ineffective because a failure in the integrity of the coating would result in accelerated corrosion at the breach. The Warm Springs District chose a concentric pipe construction, with insulation between inner and outer pipes. This permits a corrosion resistant outer pipe to resist soil induced corrosion, while the inner pipe is protected from the soil chemistry. Insulation is used between the pipes to reduce heat loss from the system between the user and the wells.